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Studies on the Biosynthesis of Porphyrin and Bacteriochlorophyll by Rhodopseudomonas Spheroides 5

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Studies on the Biosynthesis of Porphyrin and Bacteriochlorophyll by Rhodopseudomonas Spheroides 5 Biochem. J. (1964) 90, 607 607 Studies on the Biosynthesis of Porphyrin and Bacteriochlorophyll by Rhodopseudomonas spheroides 5. ZINO-PROTOPORPHYRIN CHELATASE* BY A. NEUBERGER AND G. H. TAIT Department of Chemical Pathology and M.R.C. Research Group in Enzymology, St Mary's Hospital Medical School, London, W. 2 (Received 31 July 1963) Of the metal complexes of protoporphyrin which are of biological importance those of magnesium MATERIALS AND METHODS and ferrous iron are of special importance. Mag- Chemica,1. [65Zn]Zinc chloride was obtained from The nesium protoporphyrin is most probably an inter- Radiochemical Centre, Amersham, Bucks. Tween 20 and mediate in the biosynthesis of chlorophyll (Granick, Tween 80 were gifts from Honeywell and Stein Ltd., 1961) and of bacteriochlorophyll (Gibson, Neu- London. De-ionized water, which was used throughout, berger & Tait, 1963), while ferrous protoporphyrin was prepared from distilled water by passing it through a (protohaem) is a component of haemoglobin, myo- column of Bio-Deminrolit (The Permutit Co. Ltd., Lon- don). globin, cytochromes and other haem proteins. The Ether was dried over anhydrous Na2SO4 and redistilled enzymic formation of magnesium protoporphyrin before use. has not been described, but that ofprotohaem from All other chemicals were obtained or prepared as Fe2+ ions and protoporphyrin is well documented. described by Gibson et al. (1963). Ferrochelatases have been found in mitochondria Organisms. The strain of Rhodopseudomonas spheroides from rat liver (Labbe & Hubbard, 1961) and pig and the conditions of growth have been described by liver (Porra & Jones, 1963 a, b) and in haemolysates Gibson et al. (1963). Except where stated otherwise the from duck blood (Yoneyama, Oyama, Sugita & organisms used to make enzyme extracts were grown semi- Yoshikaya, 1962) and chicken blood (Schwartz, anaerobically in the light as described by Gibson, Neu- & 1959). These enzymes berger & Tait (1962). For certain experiments organisms Hill, Cartwright Wintrobe, were grown aerobically in the dark. 'High' or 'low' con- have been partially purified and their specificities tent of 02 obtained by varying the extent of aeration gave towards various porphyrins and metal ions have unpigmented or pigmented organisms respectively (cf. been tested. They act on all porphyrins with two Lascelles, 1959). In the experiments described here the carboxyl groups. They are, however, more specific content of 02 was not measured, but this factor was towards the metal; the enzyme in duck blood controlled empirically by observing the extent of pig- (Oyama, Sugita, Yoneyama & Yoshikaya, 1961) mentation of the organisms. uses Zn2+ ions at 10 % and Co2+ ions at 2 % the rate of Fe2+ ions, the one from rat-liver mito- Enzyme sources chondria (Labbe & Hubbard, 1961) uses C02+ ions Chrornatophores. These were prepared according to more efficiently than the one in duck blood but is Gibson et al. (1963) from R. spheroides. In all experiments completely inactive with Zn2+ ions; it also uses except where stated otherwise the chromatophores were Mn2+ ions to a small extent. prepared from organisms grown semi-anaerobically in the During the course of unsuccessful experiments light in the malate medium of Lascelles (1956). designed to detect a magnesium-incorporating en- Protoptasts. These were prepared as described by Karun- zyme in chromatophores from Rhodopseudomonas airatnum, Spizizen & Gest (1958). They were lysed by spheroides the production of zinc protoporphyrin dilution of the suspension in 20% sucrose with a large volume of water. The lysed particles were collected by was noted, and the rate of its formation was greatly centrifuging, washed with 0 05M-potassium phosphate, stimulated by bringing about the reaction in a pH 7 4, and resuspended in the same buffer to a concen- water-ether emulsion. Under these conditions a tration of 10-30 mg. of protein/ml. and kept frozen at marked synthesis of zinc protoporphyrin was also -200. detected with mitochondria from a number ofmam- Mitochondria. These were prepared from guinea-pig malian tissues. The properties of these zinc-proto- liver and from rabbit liver, heart and kidney as de- porphyrin chelatases, particularly the one in R. scribed by Hogeboom (1955). After being washed twice spheroide8, are described in detail and their possible in 0*25M-sucrose the mitochondrial pellet was suspended metabolic functions discussed. in 0*05M-potassium phosphate, pH 7*4, to a concentra- tion of 10-20 mg. of protein/ml. and kept frozen at * Part 4: Gibson, Neuberger & Tait (1963). -200. 608 A. NEUBERGER AND G. H. TAIT 1964 Estimations total) in the final ether solution the activity is expressed in terms of the theoretical amount of zinc protoporphyrin Protein. The protein content of chromatophores, proto- formed in the assay. plasts and mitochondria was determined by the method of Ferrochelatase. Tris buffer, pH 8-4 (150 ,umoles), proto- Lowry, Rosebrough, Farr & Randall (1951), with crystal- porphyrin (50 ,um-moles), FeSO4 (50 ,um-moles), ascorbic line bovine plasma albumin (Armour and Co. Ltd.) as acid or GSH (10-20 Mmoles) and enzyme extract (0 5- standard. 2-0 mg. of protein) were incubated in a total volume of Porphyrin8. The concentration of porphyrins and of 0 5 ml. under N2 for 2 hr. at 37°. The treatment of the metaUoporphyrins was determined by measuring the ex- mixture followed exactly that described for the assay of tinction of solutions in ether at the absorption maximum zin¢-protoporphyrin chelatase. Since haemin is insoluble in the Soret band, and by using the appropriate molar in ether the extent of the reaction was taken as the differ- extinction coefficient. ence between the amounts of protoporphyrin recovered in Radioactivity. Solutions were plated on 6-25 Cm.2 the final ether solution when experiments were done in the aluminium planchets, dried in vacuo and counted at in- presence and absence of FeSO4 respectively. finite thinness in a Nuclear-Chicago gas-flow counter with a Micromil end window and operated at the centre of the plateau. One Mco of 65Zn gave 24400 counts/min. under RESULTS these conditions. Alternatively, solutions (0 5 ml.) were mixed with 5 ml. of a dioxan-based scintillator (Bray, Preliminary observations 1960). Radioactivity was measured in a I.D.L. scintillation The finding that the enzymic conversion of proto- counter with the upper-gate-discriminator bias set at 50 v. porphyrin, but not of magnesium protoporphyrin, Under these conditions 1 MC of 65Zn gave 253800 counts/ into the corresponding ester was min. monomethyl inhibited EDTA that Radioactive spots on paper chromatograms were located strongly by suggested proto- by radioautography. porphyrin had first to be converted into a metallo- Infrared-absorption spectra. Porphyrin (50-100 Mg.) in a porphyrin before it could be methylated (Gibson et smal volume of ether was mixed with 6 mg. of powdered al. 1963). That the insertion of the metal was KCl and the suspension was evaporated to dryness. The catalysed by an enzyme was indicated by the fol- resulting powder was pressed to form a micro-disk. The lowing experimental results. Protoporphyrin and infrared spectrum was measured on a Unicam SP. 200 chromatophores in tris buffer, pH 8f4, were in- spectrophotometer. cubated at 370 for periods up to 2 hr., after which EDTA was added, followed by S-adenosyl[Me-14C]- Assay of enzyme activities methionine. The mixture was incubated for a Zinc-protoporphyrin chelatase. To incubation mixtures further 2 hr. The amount of radioactivity incor- (vol. 0 5 ml.; cf. legends to Figs. 1-3 and Table 1) was porated into porphyrin was proportional to the added 4 ml. of acetone-0- N-NH3 (9: 1, v/v). After centri- duration of the first incubation in the absence of fuging, the supernatant was extracted twice with 3 ml. of EDTA. The formation of metalloporphyrin was also light petroleum (b.p. 60-80°). This removes the bacterio- the of radio- chlorophyll and carotenoids of the chromatophores. To the pH-dependent; greatest incorporation activity into porphyrin took place when the chro- aqueous phase, containing the porphyrins, were added 0-3 ml. of saturated NaCl, 0-5 ml. of 0-5M-KH2PO4 and matophores and protoporphyrin were incubated at 4 ml. of ether. After thorough mixing the ether layer was pH 8S4 before addition of EDTA. A large-scale removed and analysed for porphyrin and zinc porphyrin. experiment was performed in which protoporphyrin, This extraction method is modelled on that of Granick S-adenosyl[Me-14C]methionine and chromatophores (1961). In assays with 65ZnC12 this ether solution was from R. spheroides, and tris buffer, pH 8*4, were washed with 3 ml. of water before portions of it were taken incubated for 4 hr. After incubation, the por- for measurement of radioactivity. Most of the assays were phyrin mixture was extracted and purified by the done with protophorphyrin as substrate and therefore the method of Granick (1961), which does not hydrolyse extinction of the was at ether solution measured 404 m,u acid-labile This mix- and 415 mMu, which are the absorption maxima of proto- metalloporphyrins. porphyrin porphyrin and zinc protoporphyrin respectively. When ture was chromatographed on paper in the luti- other porphyrins were used as substrates the wavelengths dine-ammonia system of Granick (1961). All the at which readings were measured were chosen accordingly. radioactivity was located in an area corresponding From the known molar extinction coefficients in ether of to a porphyrin with one free carboxyl group. This protophorphyrin (404 mM, 1-35 x 105; 415 mM, 0 94 x 105) portion of the paper was cut out and the eluted and zinc protoporphyrin (404 mp, 0-75 x 105; 415 mrI, porphyrin was identified as a metalloporphyrin on 2-40 x 105) the concentration of each component was calcu- the basis of its spectrum in ethereal solution which, lated in a manner similar to that described by Dresel & in the visible region, had peaks of almost equal Falk (1956).
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